Very general question. Tank crash

[Maridia]

Hello everyone. I had a question on when tanks crash. I believe this happens to both fresh and salt water. But what is the cause? Is your tank considered crashed when your ph stays stable (let's say 8.4ph), then suddenly over night all your fish are dead! You check ph and it's 6.0ph? What makes a tank crash? What does it mean when your tank crashes? Does regular WCs help prevent crashes? How to prevent crashes?


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